1. Field of the Invention
The present invention relates to a correlation detector for outputting a degree of autocorrelation of a digital signal obtained by sampling and quantizing an analog signal, where a degree of autocorrelation corresponds to the number of consecutive samples which have a substantially identical quantized level.
2. Related Background Art
A correlation detector for detecting a degree of correlation of an input signal has generally a circuit structure such as shown in FIG. 7. An input signal is sampled by a sampling pulse having a frequency F, and a presence/absence of autocorrelation between data of the input signal at one timing and at a timing immediately before the one timing is detected by an autocorrelation detector circuit 1. A presence of "autocorrelation" between two data samples means that two data samples have a substantially identical quantized level. An absence of autocorrelation between them means that they have different quantized levels. In accordance with the detection results, an autocorrelation counter 2 counts the number r of consecutive autocorrelated data items (i.e. the number r of consecutive samples having a substantially identical quantized level) before one timing of the input signal. A value D0 corresponding to the output r of the autocorrelation counter 2 is outputted as a correlation value.
The operation of the conventional correlation detector shown in FIG. 7 will be described. The autocorrelation detector circuit (SREL) 1 is supplied with an input signal IN sampled at a clock ck such as shown in FIG. 6A. A presence/absence of autocorrelation between data of the input signal at one timing and at a timing immediately before the one timing is detected, and the detection result is outputted as a correlation presence/absence signal f (FIG. 6B) which is inputted to the autocorrelation counter 2. The length of the H level of the signal f is outputted as the count signal r (FIG. 6D) from the autocorrelation counter 2 which is inputted to an encoder 5. The encoder 5 outputs D0=0 for 0.ltoreq.r.ltoreq.1, D0=1 for 2.ltoreq.r.ltoreq.4, and D0=2 for 5.ltoreq.r (see FIG. 6E).
A clock signal CK is shown in FIG. 6C. There is a one clock delay when D0 is outputted. Therefore, the input signal IN is delayed by one clock to obtain a delayed input signal IND (FIG. 6F) which is compared with the signal D0. At a timing .alpha.1 of the signal IND, three consecutive autocorrelated data items are present before the timing .alpha.1, and so r=3 at .alpha.1. Five consecutive autocorrelated data items are present before a timing .alpha.2, and so r=5 at .alpha.2. The signal r is inputted to the encoder 5 which outputs a value corresponding to the value r. In this case, D0=1 at .alpha.1, and D0=2 at .alpha.2.
An output D0 of the conventional correlation detector is a value corresponding to the value r which is the number of consecutive autocorrelated data items before one timing of the input signal, and is not the number of consecutive autocorrelated data items after one timing of the input signal. Therefore, a correlation value obtained by the conventional correlation detector is not natural from the viewpoint of senses.